body{
    margin:0px;
    padding:0px;
    font-family:sans-serif;
    background: url(back.png);
    background-size:cover;
}
.box{
    position:absolute;
    top:50%;
    left:50%;
    transform:translate(-50%,-50%);
    /*实现块元素百分比下居中*/
    width:450px;
    padding:50px;
    background: rgba(0,0,0,.8);
    box-sizing:border-box;
    box-shadow: 0px 15px 25px rgba(0,0,0,.5);
    border-radius:15px;
}
.item{
    width: 340px;
    height: 60px;
    background: #eeeeee;
}
.item div{
    width: 170px;
    height: 60px;
    display: inline-block;
    color: black;
    font-size: 18px;
    text-align: center;
    line-height: 60px;
    cursor: pointer;
}

.box h2{
    margin:0 0 30px;
    padding:0;
    color: #fff;
    text-align:center;
}
.box .inputbox{
    position:relative;
}
.box .inputbox input{
    width: 100%;
    padding:10px 0;
    font-size:16px;
    color:#fff;
    letter-spacing: 1px;
    margin-bottom: 30px;
    border:none;
    border-bottom: 1px solid #fff;
    outline:none;
    background: transparent;
}
.box .inputbox label{
    position:absolute;
    top:0px;
    left:0px;
    padding:10px 0;
    font-size: 16px;
    color:#fff;
    pointer-events:none;
    transition:.5s;
}
.box .inputbox input:focus ~ label,
.box .inputbox input:valid ~ label
{
    top:-18px;
    left:0;
    color:#03a9f4;
    font-size:14px;
}
.box input[type="submit"]
{
    background: transparent;
    border:none;
    outline:none;
    font-size: 16px;
    color:white;
    background: black ;
    padding:15px 20px;
    cursor: pointer;
    border-radius:10px;
}